Hybrid vehicles using a combination of an internal combustion engine and an auxiliary drive source, such as an electric motor, are becoming more and more popular due to their efficient use of energy. The internal combustion engine can be operated intermittently to provide power to the hybrid vehicle's driveline when needed depending on the driving conditions. In low speed driving situations the hybrid vehicle may be operated by only using the auxiliary drive source and when more power is needed the internal combustion engine supplies additional power to the driveline, for example when driving at higher speeds. For hybrid vehicles of the parallel type, both the internal combustion engine and the auxiliary drive source could drive one or more driving wheels through the hybrid vehicle's transmission unit.
When starting an internal combustion engine, usually a traditional 12 V starter system with a starter motor is used. Modern hybrid vehicles however use 48 V electrical systems, or systems with other voltages, with lithium-ion batteries. An electrical starter motor could also be used in a 48 V system for starting the internal combustion engine in different driving conditions or for cold starts. The 48 V lithium-ion type of batteries are however in cold weather conditions limited in power, which makes it difficult to start the internal combustion engine in low temperature situations, such as in temperatures below −15° C. or even as low as −30° C., with a starter motor powered by the 48 V battery. Another complexity with an internal combustion engine at low temperatures is that the friction or drag torque at very low temperatures could exceed the maximum torque available from the starter motor in a 48 V electrical system, especially for diesel engines. Thus, cold starts at low temperatures using a starter motor in a modern 48 V hybrid vehicle electrical system could be difficult or even impossible.
One common solution to this problem is to use a 12 V starter motor having a conventional 12 V lead battery. Another alternative for starting an internal combustion engine is to use a flywheel for starting the internal combustion engine through using energy stored in the flywheel.
U.S. Pat. No. 6,098,584A discloses a starter apparatus for an internal combustion engine including a device for rotating a flywheel that in an impulse starting method is used for starting the engine with the rotational energy stored in the flywheel. Alternatively, a direct starting method could be used, where a starter-generator is directly coupled to the engine. A changeover device switches between the direct starting method and the impulse starting method as a function of the temperature of the engine so that the impulse starting method is used at comparatively lower temperatures and the direct starting method is used at comparatively higher temperatures.
A major disadvantage with a starter apparatus of this kind is that the solution is not suitable for modern hybrid vehicles that need a very compact engine and transmission design, including both an internal combustion engine and an auxiliary drive source with the possibility to start the internal combustion engine during different driving conditions that also is suitable for cold starts in very low temperatures. There is also a risk with a starter apparatus of the above described type that hesitation occurs when switching from an electrical driving mode, where the auxiliary drive source is delivering power, to a hybrid driving mode, where also the internal combustion engine is delivering power to the hybrid vehicle.
GB 2413998 A discloses method of controlling the drive train of a hybrid vehicle, where an engine is started by an electric motor generator connected to a flywheel. The vehicle is provided with a parallel hybrid drive with a series arrangement of the engine, the motor generator and a drive gear connected to a drive axle. Controllable friction clutches are provided at the input and output sides of the motor generator respectively. In an exclusively electric mode, the flywheel is accelerated by the motor generator to build up surplus momentum, where the output clutch is controlled in a slip mode. Closure of the motor input side clutch then starts the engine.
In this solution, a separate flywheel driven by an inline motor generator is used to provide sufficient torque to start the engine of the vehicle. Since the components are arranged in series, the solution is not suitable for a compact packing of a hybrid engine. Another disadvantage is that an inline motor generator must be relatively large to be able to provide the required torque to start the engine. Further, by using a separate flywheel, additional weight is added to the system.
There is thus a need for an improved way of starting an internal combustion engine in a hybrid vehicle having a compact engine and transmission design including both an internal combustion engine and an auxiliary drive source that prevents hesitation when starting the internal combustion engine and also could start the internal combustion engine in very low temperatures, such as down to −30° C.